Matlab Transmit Capture Usrp

Transmit and Capture Waveforms with Wireless Testbench

Generate, transmit, and capture RF waveforms using NI USRP radios in MATLAB.

When to Use

• Transmitting a waveform (test tone, standard-compliant signal, custom IQ) over the air
• Capturing IQ data from a frequency band
• Loopback testing (transmit and capture on the same radio)
• Multi-antenna transmit or capture
• Continuous transmission for prolonged testing
• Background capture for long-duration acquisition

When NOT to Use

• Setting up or connecting a USRP radio for the first time — use the matlab-set-up-usrp-radio skill
• FPGA targeting workflows — see Target NI USRP Radios
• Intelligent capture with preamble/energy detection — see Spectrum Monitoring
• Multi-device synchronization — see Radio Management

Prerequisites

A saved radio configuration must exist before using any transmit/capture object. List available configurations and let the user choose which one to use:

configs = radioConfigurations;
disp(configs)

If multiple configurations exist, ask the user which one they want to work with. If no configuration exists, guide the user to set one up first (see the matlab-set-up-usrp-radio skill).

Before generating code, confirm these parameters with the user if not already specified:

• Center frequency — Accept any numeric value within the radio's supported range. Do not present a constrained list of bands; let the user type a frequency.
• Sample rate — Ask what sample rate they need. Suggest common values (30.72 MHz, 61.44 MHz, 122.88 MHz, 245.76 MHz) but accept any valid rate.

Choosing the Right Object

Default to basebandTransceiver. It supports all workflows — transmit only, capture only, or both simultaneously. Use it unless you have a specific reason not to.

Why default to basebandTransceiver: A single radio configuration can only be used by one object at a time. Since basebandTransceiver handles transmit, capture, or both, it covers the vast majority of workflows without needing to switch objects. The standalone objects (basebandTransmitter, basebandReceiver) are useful when separate scripts or applications each need independent control of one direction.

Property naming differs between object types:

The transceiver uses Transmit/Capture prefixes to distinguish directions. The standalone objects do not need prefixes since they only handle one direction.

Workflow

Step 1: Create the Radio Object

radio = radioConfigurations("MyN310");
bbtrx = basebandTransceiver(radio);

Preload option: Pass Preload=true to load the FPGA application at construction time rather than on first use. This avoids a multi-second delay on the first transmit or capture call.

bbtrx = basebandTransceiver(radio, Preload=true);

Standalone objects (only when needed for independent single-direction control):

bbtx = basebandTransmitter(radio);  % TX only
bbrx = basebandReceiver(radio);     % RX only

Step 2: Configure Properties

Configure the object before transmitting or capturing. Setting properties after object creation is valid, but some changes cause a reload delay.

bbtrx.SampleRate = 30.72e6;
bbtrx.TransmitCenterFrequency = 3.5e9;
bbtrx.CaptureCenterFrequency = 3.5e9;
bbtrx.TransmitRadioGain = 20;
bbtrx.CaptureRadioGain = 40;

Only set the properties for the direction you need. For transmit-only workflows, skip the Capture* properties (and vice versa).

If using standalone objects (basebandTransmitter / basebandReceiver), properties are unprefixed:

bbtx.SampleRate = 30.72e6;
bbtx.CenterFrequency = 3.5e9;    % not TransmitCenterFrequency
bbtx.RadioGain = 20;              % not TransmitRadioGain

Property guidance:

Step 3: Prepare the Transmit Waveform

The waveform must be a complex column vector (single antenna) or complex matrix (multi-antenna, one column per antenna). Values must be normalized to the range [-1, 1].

Test tone:

numSamples = 30720;
t = (0:numSamples-1)' / bbtrx.SampleRate;
txWaveform = 0.8 * exp(1j*2*pi*1e6*t);

Random OFDM-like signal:

numSamples = 30720;
txWaveform = complex(randn(numSamples,1), randn(numSamples,1));
txWaveform = 0.7 * txWaveform / max(abs(txWaveform));

Load a pre-generated waveform from file:

waveStruct = load("myWaveform.mat");
txWaveform = waveStruct.waveform;
txWaveform = 0.8 * txWaveform / max(abs(txWaveform));

Use this pattern with waveforms generated by 5G Toolbox, LTE Toolbox, WLAN Toolbox, or any custom signal generation workflow.

Waveform requirements:

Pass TransmitDataType to the constructor when using Preload=true:

The FPGA application is configured for a specific transmit data class at construction time. Without preload, the class is inferred from the waveform on the first transmit call. With preload, the transceiver has no waveform to infer from, so pass TransmitDataType explicitly — otherwise the FPGA reloads on the first transmit call and defeats the point of preloading.

bbtrx = basebandTransceiver(radio, Preload=true, TransmitDataType="double");
% ... configure frequency/gain/sample rate ...
transmit(bbtrx, txWaveform, "continuous");   % waveform class must match TransmitDataType

Step 4: Transmit

Once (single-shot) — transmit the waveform exactly once:

transmit(bbtrx, txWaveform, "once");

The radio transmits the waveform one time and then stops automatically. Use for pulsed or one-shot testing. Add a few redundant samples to the end of the waveform for reliability in this mode.

Continuous — transmit repeatedly until stopped:

transmit(bbtrx, txWaveform, "continuous");

The waveform loops continuously on the radio until stopTransmission is called. Use for:

• Sustained interference or signal generation
• Loopback testing where capture timing is decoupled from transmit
• Over-the-air tests where a receiver needs a persistent signal

Stop continuous transmission:

stopTransmission(bbtrx);

Step 5: Capture

Foreground capture (blocks MATLAB until complete):

[data, timestamp, droppedSamples] = capture(bbtrx, milliseconds(10));

The length argument accepts a duration value (e.g., seconds(1), milliseconds(10)) or a sample count as a positive integer.

Output arguments:

CaptureDataType defaults to int16 — cast before FFT, filtering, or arithmetic:

The captured data is a complex vector/matrix whose class matches CaptureDataType. The default is "int16" (fixed-point) to minimize memory. Most MATLAB signal-processing functions — including fft, abs, filter, bandpass, pwelch, and element-wise math — do not accept complex int16 and will error at runtime. Handle this in one of two ways:

% Option A — set the property before capture (all downstream code sees double)
bbtrx.CaptureDataType = "double";
[data, ~, dropped] = capture(bbtrx, milliseconds(10));
X = fft(data);   % works

% Option B — cast after capture (keep the memory savings during transfer)
[data, ~, dropped] = capture(bbtrx, milliseconds(10));
data = double(data);
X = fft(data);   % works

Use Option A by default. Only prefer Option B when memory during capture is tight and you want the compact int16 payload until processing begins.

Background capture (non-blocking):

Use background capture when:

• Capture duration is long (> a few seconds) and MATLAB must remain responsive
• You want to do post-processing, display updates, or other work during acquisition
• You are combining capture with continuous transmit and need to monitor both

Use foreground capture (the default) when:

• Capture is short and you need the data immediately for the next step
• The script is linear (no concurrent work needed)

Choosing the right pattern:

Background capture with polling:

capture(bbtrx, seconds(30), Background=true);

% Poll until complete
while isCapturing(bbtrx)
    pause(1);
end

% Retrieve results
[data, timestamp, droppedSamples] = captureOutputs(bbtrx);

Background capture with callback (no polling needed):

capture(bbtrx, seconds(30), Background=true, ...
    CompletionFcn=@(data, ts, dropped) handleCapture(data, dropped));

Long capture to file (background + SaveLocation + direct-to-host):

For captures that exceed onboard buffer or RAM, combine all three options:

capture(bbtrx, seconds(60), ...
    Background=true, ...
    SaveLocation="captured_data.mat", ...
    UseRadioBuffer=false);

while isCapturing(bbtrx)
    pause(5);
end
filePath = captureOutputs(bbtrx);
fprintf("Saved to: %s\n", filePath);

When SaveLocation is specified, captureOutputs returns the file path instead of loading data into the workspace.

Stop a background capture early:

stopCapture(bbtrx);
[data, timestamp, droppedSamples] = captureOutputs(bbtrx);

UseRadioBuffer — choosing between radio buffer and direct-to-host:

The UseRadioBuffer name-value argument controls where captured samples are stored during acquisition. The right choice depends on capture length, sample rate, and host network capability.

% Radio buffer (default) — data is stored in onboard radio memory, then
% transferred to the host after capture completes. Most reliable option.
[data, ~, dropped] = capture(bbtrx, milliseconds(100));

% Direct-to-host — data streams continuously from radio to host over the
% network during the capture. Required when capture exceeds onboard memory.
[data, ~, dropped] = capture(bbtrx, seconds(30), UseRadioBuffer=false);

Use UseRadioBuffer=true (default) when:

• Capture length fits within the radio's onboard memory
• You want the most reliable capture (no dependency on sustained host throughput)
• Sample rate is high and you cannot tolerate any drops

Use UseRadioBuffer=false when:

• Capture length exceeds the radio's onboard buffer capacity (see table below)
• Using USRP X310 with TwinRX daughterboard capturing more than 2 antenna channels
• You need arbitrarily long captures (minutes or hours of data)

Onboard buffer capacities (determines when direct-to-host is required):

Direct-to-host performance considerations:

Direct-to-host capture requires sustained network throughput for the entire capture duration. The maximum achievable sample rate depends on host and network configuration and varies between runs depending on system load. If drops occur, either reduce the sample rate, reduce the number of antennas, or run radioSetupWizard to optimize host network settings for your platform.

Evaluating host performance: To find the maximum sustainable direct-to-host rate, iterate captures at decreasing sample rates with DroppedSamplesAction="none" until one succeeds:

bbrx.DroppedSamplesAction = "none";
sampleRates = 245.76e6 : -10e6 : 10e6;
for idx = 1:numel(sampleRates)
    bbrx.SampleRate = sampleRates(idx);
    [~, ~, dropped] = capture(bbrx, 2*2^28, UseRadioBuffer=false);
    if ~dropped
        fprintf("Max sustained rate: %.1f MHz\n", sampleRates(idx)/1e6);
        break
    end
end

Stop a background capture early:

stopCapture(bbtrx);
[data, timestamp, droppedSamples] = captureOutputs(bbtrx);

Step 6: Loopback Test (Transmit + Capture on Same Radio)

A loopback test verifies the full TX/RX chain using basebandTransceiver. Set TransmitCenterFrequency and CaptureCenterFrequency to the same value so the signal couples internally.

Method A: Continuous transmit then capture

Start continuous transmission, allow a brief pause for the radio front-end to stabilize, then capture.

radio = radioConfigurations("MyN310");
bbtrx = basebandTransceiver(radio, Preload=true);
bbtrx.SampleRate = 61.44e6;
bbtrx.TransmitCenterFrequency = 2.4e9;
bbtrx.CaptureCenterFrequency = 2.4e9;
bbtrx.TransmitRadioGain = 10;
bbtrx.CaptureRadioGain = 30;

% Generate test tone
numSamples = 61440;
t = (0:numSamples-1)' / bbtrx.SampleRate;
txWaveform = 0.8 * exp(1j*2*pi*1e6*t);

% Transmit continuously, pause for stabilization, then capture
transmit(bbtrx, txWaveform, "continuous");
pause(1);
[rxData, ~, droppedSamples] = capture(bbtrx, milliseconds(10));
stopTransmission(bbtrx);

% Verify
if ~droppedSamples
    fprintf("Loopback OK: captured %d samples, no drops.\n", size(rxData,1));
else
    warning("Samples were dropped during loopback.");
end

% Visualize
sa = spectrumAnalyzer(SampleRate=bbtrx.SampleRate);
sa(rxData);

Step 7: Multi-Antenna Configuration

For MIMO or multi-channel operation, set antenna properties to arrays and provide a matrix waveform.

radio = radioConfigurations("MyX410");
bbtrx = basebandTransceiver(radio);
bbtrx.SampleRate = 61.44e6;

% Configure 2 TX and 2 RX antennas
bbtrx.TransmitAntennas = ["DB0:RF0:TX/RX0", "DB0:RF1:TX/RX0"];
bbtrx.CaptureAntennas = ["DB0:RF0:RX1", "DB0:RF1:RX1"];
bbtrx.TransmitCenterFrequency = [3.5e9, 3.5e9];
bbtrx.CaptureCenterFrequency = [3.5e9, 3.5e9];
bbtrx.TransmitRadioGain = [15, 15];
bbtrx.CaptureRadioGain = [35, 35];

% Waveform: N×2 matrix (one column per TX antenna)
numSamples = 61440;
t = (0:numSamples-1)' / bbtrx.SampleRate;
tx1 = 0.7 * exp(1j*2*pi*1e6*t);
tx2 = 0.7 * exp(1j*2*pi*2e6*t);
txWaveform = [tx1, tx2];

transmit(bbtrx, txWaveform, "continuous");
[rxData, ~, dropped] = capture(bbtrx, milliseconds(10));
stopTransmission(bbtrx);

% rxData is N×2: one column per RX antenna
fprintf("Captured %d samples on %d antennas.\n", size(rxData,1), size(rxData,2));

Antenna naming varies by device — TX and RX ports have different names:

Assign an invalid value to see the full list of accepted antenna names for your device.

Troubleshooting

Key Functions Reference

Conventions

• Default to basebandTransceiver. Use it for transmit-only, capture-only, or both. Only use basebandTransmitter/basebandReceiver when a separate script needs independent single-direction control.
• Normalize waveforms. Always scale to peak magnitude ≤ 0.8 to leave headroom and avoid DAC clipping.
• Start with low gain. Begin TX gain at 10–20 dB and RX gain at 30–40 dB; adjust based on observed signal levels.
• Continuous transmit before capture. For loopback, start transmit(..., "continuous") first, then capture. This ensures the signal is present when capture begins.
• Always stop transmission. Call stopTransmission when done to release radio resources.
• Check dropped samples. Always inspect the third output of capture — it is logical (true = drops occurred). If true, run radioSetupWizard to optimize host settings or reduce sample rate.
• Wait before retrieving outputs. Never call captureOutputs until isCapturing returns false or stopCapture has been called. Calling it while capture is still running will error.
• Use duration for capture length. Prefer milliseconds(N) or seconds(N) over raw sample counts for readability and portability across sample rates.
• Clean up objects. Call clear on radio objects when finished to release hardware for other applications.

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